1. Field of the Invention
The present invention relates to systems and methods for signal transmission and reception, and in particular to a system and method for steering a phased array antenna beam in moving vehicles.
2. Description of the Related Art
Phased array antennas (PAAs) are used in a variety of fixed and mobile applications including spacecraft, aircraft, naval vessels, and terrestrial vehicles. One of the advantages of PAAs is that they permit electronic beam steering, which is typically more reliable and permits faster slew rates than mechanically steered antennas. To steer the beam, PAAs typically include an Electronic Beam Steering Control (BSC) unit, located within the aperture enclosure, to accept antenna beam steering commands from a central host, to calculate the phase shift values required to steer the antenna according to the commands, and to load this phase shift values into each element/phase shifter.
Many mobile platforms include communication systems that are equipped with multiple PAAs, mounted on multiple surfaces or faces of the platform. These faces can often move independently, due to flexure in the vehicle body, masts or other structures to which the PAA is mounted. This can result in significant challenges to the tracking algorithm employed by the host to generate the antenna beam steering commands. Such challenges include substantially increased throughput, because the host must generate and provide correction to multiple pointing vectors, while responding to the independent dynamics of the multiple PAA/faces, which can include high frequency vibration and oscillation modes.
The ability for Phased Array Antenna (PAA) systems installed on mobile platforms to accurately acquire, track and communicate with moving targets must often be performed in the absence of any recognizable radio frequency (RF) power detection or demodulated Receive Signal Strength Indicator (RSSI) feedback. Thus, open loop pointing must be used to keep the beam on target.
Conventional system design relies solely on centralized Inertial Navigation Systems (INS) for open loop beam pointing. An initial pointing vector is determined through knowledge of the target's general location or through a search/acquire algorithm. The beam is then kept somewhat on-target, within the capabilities of the system, using the INS to keep track of the changes to the platform attitude. The acquisition and tracking is typically performed in some centralized (host) controller, which then passes the corrected pointing vector to the PAA.
The on-board INS is highly accurate but provides updates at a relatively slow rate . . . in most cases below 100 Hz. While this is generally acceptable for large aircraft in non-turbulent flight (when vehicle's motion is much less than 20°/sec), and interpolation can be used to derive data in-between updates, this is not the case when the platform is moving at higher rates. In some applications, it is not uncommon to experience angular rates of 300°/sec or more. Since the update rate is approximately 100 Hz, data latency alone will cause angular errors of three degrees. Further exacerbating the problem, secured communication in mobile network operations requires highly directional PAA systems, which increase pointing accuracy requirements in the order of one degree or better. Fast moving vehicles maneuvering in trenched terrain may also encounter unexpected maneuvers.
Even if INS systems on the host platform were of sufficient bandwidth, they would still be incapable of providing the data required to accurately direct the PAA beam. That is because PAA antennas may also be mounted on appendages that flex with respect to host vehicle (e.g. a PAA mounted on a tall mast of a ship at sea).
In any of the foregoing situations (high angular rate motions of the host platform, movement of the PAA relative to the host platform, or flexure of the PAA itself), can cause mobile communication to be interrupted.
There is a need to provide a beam pointing system that ameliorates the foregoing difficulties. The present invention satisfies this need.